<!DOCTYPE html>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <title>Title</title>
    <style>
        .c1{
            background-color: red;
            height: 200px;
            width: 200px;
        }
    </style>
</head>
<body>

<div class="c1"></div>


<script src="jquery.js"></script>
<script>
    // $('.c1')[0].onclick = function () {
    //     this.style.backgroundColor = 'green';
    // }

    $('.c1').click(function () {
        $(this).css('background-color','green');
    })

</script>
</body>
</html>